1. Field of the Invention
The present invention relates generally to a keyless entry system for locking and unlocking a vehicular lock device, such as a vehicular door lock device, utilizing a pocket portable radio signal transmitter. More specifically, the invention relates to an antenna arrangement suitable for communication with the pocket portable radio signal transmitter and a vehicle mounted controller.
2. Description of the Background Art
Keyless entry systems which allow keyless or remote control operation for vehicular devices, such as a vehicular lock device, utilizing a credit- or bank-card like pocket portable radio signal transmitter, have been disclosed in the U.S. Pat. No. 4,719,460 issued on Jan. 12, 1988, to Mikio Takeuchi et al, commonly assigned to the assignee of the present invention. In such a keyless entry system, radio wave signals containing a transmission request command and an authorized transmitter identifying code, are transferred between the transmitter and a controller which is mounted on the vehicle via an antenna. In practice, radio signal transmission between a transmitter antenna and a vehicle mounted antenna is performed by electromagnetic induction.
In order to assure radio transmission, the U.S. Pat. No. 4,670,746, issued on Jun. 2, 1987, to Yoshiyuki Taniguchi et al, also assigned to the common assignee to the present invention, discloses a double loop antenna arrangement.
In such prior proposed antenna arrangements, the antennas are mounted outside of the vehicle body.
In the keyless entry system which has been practically implemented, a loop antenna is installed within a door mirror unit as disclosed in the co-pending U.S. patent application Ser. No. 895,371, filed on Aug. 11, 1986, which corresponds to German patent First publication No. 36 27 193. In this construction, in some situations the transmitter can be located at a relatively distant position from the manually operable command switch. When the distance between the transmitter and the door mirror mounted loop antenna is relatively wide, electromagnetic induction for transferring the signals cannot be assured.
On the other hand, aforementioned U.S. Pat. No. 4,670,746 proposes the antenna arrangement having a vehicle mounted antenna printed on a side window. The U.S. Pat. No. 4,670,746 further discloses a double loop layout of the antenna by arranging one antenna loop on the side window and another antenna loop on the door mirror. Such a double loop antenna is successful to assure transmission of a radio signal between the radio signal transmitter irrespective of the radio signal phase. On the other hand, this arrangement encounters a problem in that the antennas to be mounted on the side window and the door mirrors are to be prepared and installed separately from each other. Furthermore, the loop antenna mounted on the door mirror creates the same problem as that in the aforementioned U.S. Pat. No. 4,670,746.